We have two 3 node clusters in separate data centers, let's call them ClusterP and ClusterS.
ClusterP is our primary and ClusterS is our secondary for disaster recovery. Each cluster has multiple failover cluster instances on it but they are completely segregated from each other.
We'd like to introduce AlwaysOn Availability Groups from ClusterP to ClusterS. I've read multiple guides to this and there is one question I can't really answer. We do NOT have the ability to have cross-datacenter shared storage (no SAN mirroring across datacenters). With this setup, is it possible to setup AlwaysOn Availability Groups?
I understand if we had a standalone server in the secondary datacenter, we could setup an AG with an FCI by adding that server as a node in WFCS, removing it as an owner from any of the FCIs on ClusterP, and setting it up as an AG member. This method does not use shared storage based on my research.
My confusion comes from having two-three node clusters and how they can work together with AGs. It seems to me I need to disband the cluster on ClusterS, add the nodes to ClusterP, add the FCIs needed then setup the nodes so that the physical nodes in location of ClusterS can only host those FCIs and the nodes in location of ClusterP can only host those FCIs. Will this work?
I'd appreciate any tips.
Best Answer
No, that is not possible. Outside of a migration, an availability group cannot straddle multiple failover clusters (WSFC).
If you want to leverage an AG, you'll have to ensure that all participating replicas reside within the same WSFC cluster. As for SQL Server failover cluster instances participating in the AG, that is not a problem. A FCI can be an AG replica.
You can achieve a geographically dispersed cluster with a few different strategies. If you need to have a witness vote (which, if you have three node clusters then that wouldn't help and you'd be best with a node majority quorum model) you could utilize a file share witness.
If what you're looking for is your availability group to span two data centers, then yes that is possible. The only requirement for this situation would be a single WSFC cluster (which is possible to span multiple data centers).